It’s tough at the top. No matter how powerful and well-established you are, there’s always some pretender trying to knock you off your perch. Even if you have an entire kingdom at your beck and call, and a suitably spiky metal throne to sit on, you probably aren’t secure.

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We don’t know what the new top predator will be once the Arctic summer sea ice has disappeared – something that could happen within decades. But the little-known Greenland shark is a good candidate. It is the largest fish in the Arctic. When the Greenland shark gets hungry, even beluga whales may find themselves on the menu.

Polar shark

Despite its size, we don’t know much about the Greenland shark. That’s partly because it lives in the remote Arctic, often swimming deep beneath the ice sheets, which makes it rather hard to find.

M. Aaron MacNeil of the Australian Institute of Marine Science in Townsville, Queensland and colleagues have pulled together what little is known.

Greenland sharks are perhaps best known as the main ingredient in hákarl, an Icelandic speciality prepared by burying the shark in gravel for several months to ferment the flesh. The result stinks of ammonia and many people struggle to keep it down, as this video illustrates. One blogger sampled it and wrote&colon; “Hákarl is in fact a good approximation of the sound your throat makes as it contorts and constricts in a desperate attempt to regurgitate the chunks of fetid fish.”

One of the largest shark species, Greenland sharks grow from around 40 centimetres at birth to over 5 metres. One clocked in at 6.4m long and weighed 1023 kilograms. They grow slowly, partly because they live in such cold water, so the largest sharks may be more than 100 years old.

On the prowl

Greenland sharks are one of the top Arctic predators. They mostly eat fish, but will also take invertebrates like squid, crabs and jellyfish. From time to time they also hunt marine mammals like seals.

There are hints that they sometimes go for much larger animals, but it’s hard to be sure whether they hunt them or simply scavenge their corpses. Remains of northern minke whales – adults of which are at least 7m long – have been found in the stomachs of Greenland sharks.

But in one case it seems clear that the sharks are actively hunting. MacNeil’s team has seen live beluga whales that have large circular bitemarks. The bites are the same size as the mouths of Greenland sharks, and no other Arctic species could have made them. Belugas must regularly surface to breathe, often finding small cracks in the sea ice, and the sharks may strike them there.

The sharks’ hunting skills are particularly impressive, because most of them are at least partially blind. They are regularly parasitized by tiny crustaceans called Ommatokoita elongata, which attach themselves to the sharks’ eyes and severely damage them. In many populations, 75-100 per cent of sharks are parasitized. But it may not matter that they’re half-blind, perhaps because the sharks hunt in dimly-lit waters where smell is more important.